In recent years, hydrometallurgical processes for the extraction of copper from ore have supplanted traditional pyrometallurgical methods. Pyrometallurgical methods involve the use of fire to extract pure metals from ore by smelting. Increased awareness of the environmental impact of these processes however have lead to an increase in environmental standards and the concomitant increase in the capital cost and operating cost of smelter equipment. Hydrometallurgical techniques have thus become a preferred method for the extraction of copper from copper ores.
Hydrometallurgical techniques involve the extraction or leaching of copper from copper ores into aqueous leach solutions. Ore is treated with aqueous solutions which dissolve the copper from the ore. Pure copper is then recovered from the leach solutions by solvent extraction and then electrowinning. Electrowinning is a process in which copper is plated onto an electrode from an aqueous solution containing high concentrations of isolated copper ions.
An efficient leaching system uses dilute solutions of sulfuric acid to extract copper from copper oxide and oxide/sulfide containing ores. Ferric (iron(III)) ions are often added to the acid solutions to improve the efficiency of the leaching process by oxidizing the copper (I) to its more soluble copper (II) form and sulfide to sulfur. The resulting leach solutions contain not only soluble copper and sulfate at low pH but they also contain a variety of metals including iron, manganese, aluminum, magnesium and molybdenum of which high concentrations of iron is the primary concern. Another efficient leaching system is chloride leaching. Chloride leaching is particularly effective at leaching copper from sulfide containing copper ores such as chalcopyrite. In this process ferric chloride is used as the source of chloride ions to complex the copper and to oxidize the copper (I) to copper (II) contributing to a high iron concentration in the leach solution.
The copper concentrations in the leach solutions can range from about, 1 gram/liter (g/L) to about 50 g/L with typical concentrations falling between 1 g/L and 6 g/L. The pH of these solutions ranges between 1.2 and 2.2 pH units. Copper concentrations in a typical electrowinning tank however are between about 30 g/L and 35 g/L. It is therefore necessary to extract and concentrate the copper in these mixed acid leaches before purifying the copper by electrowinning.
The technology most widely used to extract and concentrate the copper in these mixed acid leach solutions is solvent extraction. In the solvent extraction process water soluble heavy metal salts are complexed with organic ligands to produce low polarity or neutral charge complexes that have limited solubility in water but are highly soluble in an organic solvent (organic phase) which is immiscible with water. Ligands such as aryl-hydroxyoximes are used for this purpose. These ligands selectively bind to the copper ions in the mixed metal solutions to create a charge neutral copper complex. These complexes are soluble in an organic solvent. In this way copper is selectively transferred into the organic phase. Salts can also be added to the aqueous phase to force the metal complexes into the organic phase. Other variations of this process incorporate ligands which form micelles in the aqueous phase or ligands which stay in the organic phase and are polar on one end and so are drawn to the aqueous-organic interface where they react with the metals to form non-polar organic soluble complexes soluble in the organic phase. When the organic phase becomes saturated with relatively high concentration of complexed copper, the copper must be decomplexed and released into the clean aqueous phase for final electrowinning. Some of the problems associated with the solvent extraction process include the necessity to execute multiple wash steps of the organic phase when chloride leach solutions are being treated; the need to add equilibrium modifiers to facilitate the uptake into and release of copper from organic phase and the formation of “crud” that forms at the aqueous-organic phase interface causing equipment fouling. Organic solvent and ligand loss further complicates the process. The primary drawback with this technology however is the organic phase. The solvents used in the organic phase include benzene, toluene, chloroform, hexanes and octanes among others. Kerosene is the solvent of choice for most large scale production mining operations. These solvents are typically toxic, flammable, and have adverse environmental impacts. Solvent loss during the extraction process extracts a negative economic toll. Ligand and equilibrium modifier loss is also an environmental and economic problem associated with the solvent extraction process.
An alternative to the solvent extraction processing of hydrometallurgical solutions is the use of swellable resin beads as an extraction medium. Swellable resin beads have a long history as the matrix on which ion exchange and metal ion chelation technologies are built. These swellable resin beads are generally lightly crosslinked polystyrene which is modified to accommodate the addition of pendant ion exchange or chelating ligands and occasionally other groups to decrease the hydrophobicity of the polymer. Copper selective ligands have been chemically bonded to polystyrene beads for the processing of copper leaches. There are however problems associated with using swellable resin bead technology in high throughput operations. Lightly crosslinked polystyrene beads are highly porous and the extractant ligands are bound throughout the polymer matrix. Many, if not most, of the ligands are buried deep within the polymer bead. The feed solution must diffuse through the bead to reach these sites for extraction to take place. The required process flow rates are often much faster than the rate of diffusion through the resin bead. For this reason a material that may have a high capacity in a batch application where the extractant and the feed solution have long contact times has greatly reduced capacity in flow applications. Because of the porous nature of the resin beads they also have a tendency to collapse when subjected to the pressures generated by the fast moving solution in a column application. The beads at the exit end of the column flatten and pack more tightly together which in turn causes an increase in the backpressure of the system. This causes a decrease in metal ion capacity and necessitates periodic backwashing of the column thereby limiting the useful lifetime of the material. In addition, chloride is often present in the leach solution and will contaminate the strip solution. This contamination is not compatible with electrowinning. This is a problem with both the solvent extraction process and currently employed resin technologies.
From the foregoing it is apparent that the technical challenge posed in the recovery of high grade copper from low grade ore is to devise an efficient, environmentally safe method of selectively extracting copper from low concentration, low pH leach solutions to produce high concentration, high purity aqueous solutions suitable for electrowinning (i.e. free of ferric and chloride ions).